On 8/9/07, Chad Scherrer <chad.scherrer at gmail.com> wrote:
>> extract :: [Int] -> [a] -> [a]
> extract = f 0
> where
> f _ _ [] = []
> f _ [] _ = []
> f k nss@(n:ns) (x:xs) = if n == k then x:f (k+1) ns xs
> else f (k+1) nss xs
Finally, even if no one else is using it, it would be good to settle
> on reasonable names for things more easily. Is there a better name for
> this function? Is there a reason not to call it "extract"?
Other possible names which occur to me include select, slice, mask. I think
I like 'select' best myself, but 'extract' works too.
Amusingly, extract is intimately related to function composition. Suppose we
have
listify :: (Int -> Int) -> [Int]
listify = flip map [0..]
Then if f, g :: Int -> Int, and f is monotonically increasing, we have the
identity
(listify f) `extract` (listify g) = listify (g . f)
This randomly occurred to me as I was falling asleep last night and I
thought I would share. =)
-Brent
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